Portable battery powered barcode label printers are known that use a static random access memory to store information, such as programmable software routines and/or programmable font and format data which is necessary for the intended operation of the label printer. This information generally remains fixed over numerous operations of the label printer and until the label printer is reprogrammed. This is opposed to the data to be printed which changes frequently. One such printer is described in U.S. Pat. No. 5,483,624.
However, when such a memory is used to store information that is necessary for the intended operation of the printer, a minimum voltage must be applied to the static RAM to ensure that the stored information is maintained therein and not lost or corrupted. In order to ensure that the static RAM is always powered sufficiently to maintain necessary, stored information intact therein, prior portable label printers have included a backup battery in addition to a main battery as described in U.S. Pat. No. 4,724,033.
An example of the circuitry connecting the main battery and backup battery to a memory such as a static RAM as was used in the prior art is depicted in FIG. 3. As shown therein, +Vcc, which is the regulated voltage provided from the main battery via a voltage regulator is coupled to a static RAM which is powered by +Vmem through a Shottky diode 10 wherein the anode of the diode 10 is connected to +Vcc. A second Shottky diode 12 having its cathode connected to the cathode of the diode 10 with its anode coupled to a backup battery via a resistor 14. It has been found that this circuit contributes to failure of the static RAM and/or corruption of the data therein due to the large voltage drop across the Shottky diode 10 which may be as large as 0.5V-1V. It has also been found that there is leakage current from the backup battery across the Shottky diode 10 which is undesirable and could cause a draining of the backup battery too quickly.